1. Field of the Invention
The invention is related to the field of magnetic disk drive systems and, in particular, to patterned magnetic media.
2. Statement of the Problem
Many computer systems use magnetic disk drives for mass storage of information. Magnetic disk drives typically include one or more sliders that include a read head and a write head. A suspension arm holds the slider above a magnetic disk. When the disk rotates, an airflow generated by the rotation of the disk causes an air-bearing surface (ABS) side of the slider to ride at a particular height above the disk. The height depends on the shape of the ABS. As the slider rides on the air bearing, an actuator moves the suspension arm to position the read head and the write head over selected tracks of the disk.
A conventional disk is divided into data fields and servo fields. The data fields are comprised of a plurality of data sectors where actual data is stored. In the data fields, the magnetic surface of the disk is divided into small magnetic regions, each of which is used to encode a single binary unit of information. The magnetic regions include a few dozen magnetic grains forming a magnetic dipole, which generates a highly localized magnetic field. The write head magnetizes a magnetic region by generating a strong local magnetic field to store a bit of data within the magnetic region during a write process. The read head senses the magnetic dipole of the magnetic region to read the bit of data during a read process.
The servo fields are comprised of a plurality of servo sectors that are used to assist in reading and writing to the data sectors, such as by positioning the read head and the write head over the center of tracks, reading a synchronization signal, etc. When the write process is performed on the disk, the read head and the write head are positioned over the tracks based on a positioning signal that is read from the servo sectors on the disk. The servo sectors include burst fields that are used to guide the read head and the write head to the proper position within data tracks on the disk.
As the areal density of the disk increases, the super-paramagnetic effect causes reliability problems for magnetic data storage. The super-paramagnetic effect occurs when the magnetic regions on the disk become so tiny that ambient temperature can reverse the orientation of their magnetic dipole. The result is that the bit is reversed and the data encoded by the bit is corrupted.
One solution to the problems posed by the super-paramagnetic effect is to pattern the disk. A patterned disk is created as an ordered array of discrete magnetic islands, with each island capable of storing an individual bit. Because each island represents an individual magnetic domain, the patterned disk is thermally stable and higher densities may be achieved. One consequence of using patterned disks is that the write process must be synchronized to the magnetic islands patterned on the disk. As the disk rotates and the islands continually pass underneath the write head within the slider, the frequency and phase of the write signal for the write head is timed so that the desired magnetic state is written to the islands passing directly underneath the write head. Imprecision in the synchronization of the write signals with the passage of the islands increases the probability that the islands will be written incorrectly and the data will be corrupted. Various publications describe the necessity of synchronization and contain examples of how synchronization of the write signal can be achieved, such as by Schabes, Journal of Magnetism and Magnetic Materials 320, 2880-2884 (2008) and Albrecht et al., in Nanoscale Magnetic Materials and Applications, edited by Liu et al. (Springer, Dordrecht, 2009), pp. 237-274.
The frequency and phase of the write signals can be determined by reading synchronization fields on the disk with read head. The synchronization fields contain magnetic patterns which, when read by read head, allow for the correct synchronization information to be deduced.
In current practice, the read process is halted when writing any data because the read head senses noise from the write head during the write process. One drawback to halting the read process during writing is the inability to read synchronization fields while writing to the disk. Therefore, larger tolerance budgets are utilized for synchronizing the write signal to reduce the probability that the islands on the disk will be written incorrectly. This use of larger tolerance budgets ultimately reduces the bit density of the disk, which reduces the total storage available for storing data.